The Geophysics group of the Ebro Observatory-Ramon LLull University and the Group of Analysis of Adverse Meteorological Situations of the University of Barcelona have participated in a study led by the German Research Center for Geosciences (GFZ) and published in the most prestigious scientific journal in the world, Nature. The study investigates pairs of flood or drought events that take place in the same area but are separated by a few years. It is shown that societies, after the first event, tend to decrease their vulnerability, but the changes applied are not, in general, sufficient to cope with unprecedented events.

Floods and droughts can cause severe damage and are on the rise in many parts of the world. The impact of such natural hazards can be reduced through appropriate risk management if the causes of the increasing damage are known. However, this has so far been hampered by a lack of empirical data.

A large-scale international collaborative effort by researchers from the International Association of Hydrological Sciences (IAHS), led by Heidi Kreibich of the German Research Centre for Geosciences (GFZ), has now led to important lessons from past events. A unique data set of two successive extreme flood or drought events in the same area was compiled and studied. Regions with large differences in population structure, socio-economic, climatic and hydrological conditions on all continents were studied. The analyses confirmed the assumption that appropriate risk management generally helps to reduce damage.

Pere Quintana Seguí, researcher at the Ebro Observatory, has contributed to the study, together with María del Carmen Llasat, from the University of Barcelona, ​​comparing the droughts of 1986-1989 and 2004-2008 in Catalonia, which especially they affected the Ter-Llobregat system. The first drought had a great impact and stimulated a series of improvements that reduced the vulnerability of Catalan society to these events. New reservoirs were built, drought management plans were developed, and legislation was updated to facilitate better monitoring and much more effective action. During the second event, the Barcelona desalination plant was also created, and the number of available wells was increased to make better use of groundwater in an emergency. Despite the large reduction in vulnerability, the second event was more intense than the first and the improvements applied were not sufficient to reduce the impact relative to the previous event, so the overall impact increased, in line with what happened in most of the cases analysed in the study. Thus, adaptation policies must go beyond past extremes, especially in a context of climate change.

Looking at all the paired events studied in this paper, we see that it is particularly difficult to reduce the impact of extreme events whose magnitude has not been seen in the past in the affected area. Pere Quintana explains this with two factors. First, infrastructures such as dams and reservoirs have an upper design limit up to which they are effective, but once a threshold is exceeded, they become ineffective. Second, risk management is usually introduced or adjusted reactively after major floods and droughts, while proactive, anticipatory strategies are rare. The reason for this behaviour is partly due to a cognitive bias related to the rarity and previous uniqueness of these extreme events, as well as to the nature of human risk perception: events that one has already experienced oneself are more likely to be expected again in the future.

However, two success stories were also examined, in which the damage was less despite a higher hazard in the second event. One of these stories lies in Barcelona, considered an exemplary city by the United Nations for its resilience in the face of floods. In this case, the pluvial flood episodes of September 21, 1995 and September 6, 2018 were compared. It was precisely the events of 1995 that gave the start to the improvements that took place with the Comprehensive Plan of Barcelona Sanitation and the construction of 15 rainwater reservoirs with a capacity of 477,020 m3. Three success factors were identified: effective governance of risk and emergency management, high investment in structural and non-structural measures, and improved early warning and real-time control systems. María del Carmen Llasat says: "We believe that applying these success factors can counteract the current trend of increasing damage from extreme events under climate change conditions."

Article citation: Kreibich, H., Van Loon, A.F., Schröter, K. et al. The challenge of unprecedented floods and droughts in risk management. Nature (2022).